As laser system technology improves and laser pulses continue to shorten, drilled holes, cuts, and scribed grooves that are formed on various materials by these laser systems continues to improve accordingly. Improvements are observed, for example, in recast surface residues around the edges of these features, and in the smaller taper of the features in the material being processed. Reduction and/or elimination of micro-cracks around these features also is observed.
For the same amount of average laser power delivered to a work-piece, the cost of a laser system increases dramatically as the pulse duration decreases. The cost of a CO2 laser is generally much lower than the cost of a diode pumped solid state (DPSS) laser, while the operating life time, size, weight, and reliability are comparable. For a given pulse width available from a CO2 or DPSS laser, then, the wavelength of the laser becomes the discriminating parameter (in addition to cost considerations) when attempting to obtain the most favorable holes, cuts, or grooves in the material to be processed. For example, the characteristics of these features can determine whether a mid IR (i.e., CO2 at 9.2 to 10.6 microns), near IR (i.e., DPSS at around 1 to 1.5 microns), visible (second harmonic of DPSS lasers), or UV (excimer or 3rd harmonic of DPSS lasers) is selected for performing the process. Short CO2 pulses also are of interest to the scientific community to probe the atomic and molecular relaxation rates.
Presently, the primary techniques used to obtain short laser pulses from a laser system include Q-switching, simultaneous Q-switching and Cavity-dumping, and mode-locking. Each of these short pulse generation techniques requires one or more electro-optical switches, or electro-optical modulators, to be inserted within the feedback cavity of the laser system. A cadmium tellurium (CdTe) crystal is presently the electro-optical crystal of choice for CO2 laser systems. Performing these short pulse generation techniques in CO2 lasers with CdTe crystals, however, presents problems that need to be addressed in order to maximize performance.
For example, the drive voltage for electro-optical and acousto-optical switches (or modulators) is proportional to the laser wavelength. Consequently, the modulators for CO2 lasers (i.e., operating in the 9.2 to 11 micron region) require approximately 10 times more voltage than for lasers operating, for example, in the 1 micron range. The high voltage requirement complicates the design of the electro-optical crystal holder for CO2 laser mode-locking and Q-switching applications, as it is necessary to prevent arcing and/or dielectric breakdown of the electro-optical crystal by either the high video voltage for Q-switching and/or cavity dumping, or by the RF voltage for mode-locking applications.
Further, the optical absorption of existing acousto-optical devices is too large to be inserted into a CO2 laser feedback cavity and still obtain reasonable laser efficiency. Consequently, CdTe electro-optical crystals are the present material of choice because these crystals have lower optical absorption in the 9.2 to 11 micron range than acousto-optical devices that presently use Ge as the acousto-optic medium. CdTe crystals have relatively poor thermal conductivity, however, and uniformly extracting the heat without imposing stress and causing birefringence is challenging. Further, non-uniform heat extraction can lead to spatial variations of the refractive index, which can produce an undesired bending or deflection of the laser beam.
Anti-reflection films are presently required to be deposited on the entrance and exiting surfaces of the CdTe crystal in order to reduce the optical loss within the laser cavity. These films have a low optical damage threshold, such that obtaining high laser reliability under high laser peak power and high laser pulse energy operation required for most material processing applications is difficult.
Other potential problems must be considered when addressing the heat removal from a CdTe crystal assembly. For example, the electrical resistance of CdTe crystals drops dramatically with increasing temperature, thus increasing the difficulty of controlling the temperature of the crystals. It is not uncommon for the electrical resistance to drop from 30 to 50 times the room temperature value when the crystal is operated above 50° C. Fortunately, if the crystal does not exceed 100°C, such as due to effective heat removal, the thermal resistance recovers when the crystal is returned to room temperature. Further, a CdTe crystal can easily suffer damage from RF arcing from any metal parts in the housing that are too close to the crystal, even when such metal parts are separated from the crystal by a dielectric because of the increased capacitive coupling caused by the dielectric constant of the dielectric.